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Full text of "Handbook Of Chemical Engineering - I"

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GRADING AND SCREENING                              251
coils. The rings are made this way to provide for quick discharge and reduce blinding. The outer portion of the rings is beveled and clusters of bristles fastened to a supporting arm running the length of the grader are introduced between the rings to force out any lodged material.
Material to be graded is introduced into the grader as in the ordinary revolving screen at the upper end and works its way to the lower or discharge end by gravity, the device being inclined to effect this. It will be noted that it effects a grading by one diameter or measure of volume. Its most distinctive field would seem to be for longish or spindle-shaped grains such as those of barley and wheat, which, owing to the slight difference in the short diameter cannot be graded by ordinary screens. In malting, barley is first steeped in water from 24 to 40 hr., which causes the grain to take up from 10 to 30 per cent water when it swells and begins to germinate. Following this operation the grain is piled up and the heat created by vital actions conserved, the growth being stimulated. The grain is then stirred to check the growth of the rootlets and to stimulate the growth of the acrospires. Finally the grain is dried, the germination being completely checked. A malt kiln is used for this purpose. The malt is then screened when the rootlets and acrospires fall off.
The chemical changes effected are the conversion of azotized substance into diastase, the conversion of starch into grape sugar and the imparting of color and flavor to the malt. The malt is either light or dark colored according to the amount of heat used in the kiln. When heating is prolonged more empyreumatic oil accumulates in the malt and it yields a sharper or stronger and fuller flavor. The principal advantage which would result from grading barley would be that with uniform-size berries the germination period would be about the same for all of them and the various steps in the malting process would be better defined and better regulated.
The argument for grading wheat after separatory cleaning, scouring and washing and other preparatory operations is that the wheat berry under the pressure of the breaking rolls breaks into flour, middlings and bran. The outer portion of the berry is a tough fibrous substance which is flattened out in the rolling and not broken into small particles as the other parts of the berry are. The inner portion breaks into flour and the layers between this and the outer branriy layers break into middlings, familiar to household use as " Cream of Wheat" and under other trade names. The old millers attempted to grind as fine as possible at the first break getting as much flour as possible. In modern mills gradual reduction is practiced, the second break of the first middlings after screening yielding the best household or "first patent" flour. The argument for grading the wheat would be that the berries going to a roller mill would be of uniform size and receive a uniform grind, etc. Most millers and maltsters are skeptical as to the value of ultra refinements in practice which would be introduced by close grading of barley and wheat. Other suggested uses for the disc grader is in separating pin oats and double oats from milling oats, wheat from oats, for grading paddy after hulling to avoid the serious loss in broken rice. Possibly the chief argument against the device would be its small capacity. Where the eye is to be pleased as in obtaining uniform size nuts shelled or unshelled, graders of the disc type may find application. Spoiled nuts are now separated from heavier good ones by suction pipes.
Ordinary Revolving Screens or Trommels.—The most common type of these devices is of circular section, the screen cloth or plate being supported on spiders attached to a central shaft. This type of trommel has a slope of from a fraction of an inch to an inch and a half per foot, the tendency of late years being towards the higher figure of slope. Trommels are also made conical and pyramidal with horizontal supporting shafts. There is also a type which consists of a